Structurally modified alumina supports and catalysts therefrom

Abstract: A structurally modified alumina useful as a catalyst support, or heat transfer solid for fluidized bed synthesis gas processing, comprises one or more Group IIA metals, particularly magnesium and barium, composited with particulate alumina. The composite particle is represented by formula (1), M x Al₂O 3+x ; with the core of the particle being represented by formula (2), M y Al₂O 3+y ; with M being a Group IIA metal and x and y being defined. Catalysts comprising said particulate aluminas and nickel, iron, platinum or ruthenium are referred to.

Description: This invention relates to the production of synthesis gas, or syn gas (hydrogen and carbon monoxide), from low molecular weight hydrocarbons, primarily methane, reacted in the presence of steam (steam reforming process) or oxygen (partial oxidation process), or both steam and oxygen, at high temperature within a fluidized bed of catalyst, e.g., nickel on an alpha alumina support. Suitably, particulate refractory inorganic oxide solids, notably alpha alumina solids, are admixed with the catalyst, or catalysts, of the fluidized bed as heat transfer solids to control the temperature of reaction. Processes utilizing the admixtures of catalysts and heat transfer solids in reacting low molecular weight hydrocarbons in the presence of both steam and oxygen, it has been found, have provided overall high thermal efficiencies in the production of syn gas. Certain particulate refractory inorganic oxide solids as heat transfer materials are more resistant than others to melting and agglomeration at the severe high temperature oxidizing and reducing conditions encountered in fluidized bed syn gas generation processes. These particulate refractory inorganic oxides permit substantially isothermal reactor conditions in conducting such reactions, at least so long as they resist melting and agglomeration which leads to reduced fluidization quality, gas bubbles of increased size, and inferior mass transfer. Moreover, the mechanical strength of some particulate solids is greater than others, and in any process the particles must be sufficient to withstand the stress and strain of process operations. An important pathway to loss of fluidization relates to particle degradation through mechanical attrition and break up of the particles to product fines. The amount of mechanical fracturing and attrition of the particles that can be tolerated during the operation is necessarily limited, and in any process some of the solids particles will be swept from the bed by the ascending fluidization gas, or gases. Whereas cyclone separators can be used to return major concentrations of the solids particles to the bed, no cyclone separator, or system of cyclone separators can be operated with one hundred percent efficiency. Albeit other means may exist for obtaining a more complete separation of particulate solids from the effluent, the cost of such facilities are burdensome. Hence, in most economically viable processes a significant amount of the particulate solids escapes to the atmosphere. Make up solids must therefore be added to the reactor to compensate for this loss; this loss not only representing waste, but possibly a pollution problem. Sintering and agglomeration of the fluidized bed solids have been found particularly important pathways for fluidized bed degradation, and loss of catalyst activity in fluidized bed operations. Surface impurities, notably such impurities as silicon, boron, sodium, iron, calcium and titanium, especially silicon and boron, in the outer surface of the particles, are found to promote surface melting and agglomeration of the particles in fluidized bed operations. It is believed that surface impurities form, or contribute to the formation of, surface compounds which exhibit reduced melting points relative to the normal melting point of the particulate bed solids. Hot spots, particularly as occurs in the zones of oxygen injection, contribute to and produce sintering and agglomeration of the particles. The temperatures in these zones far exceed the normally high temperature of reaction outside these zones, often by several hundred Fahrenheit degrees. Surface melting of the particles, for any reason whatever, creates a tendency of the particles to fuse, or stick together to form agglomerates; and agglomeration of the particulate solids promotes defluidization of the bed. Albeit there are a few catalyst supports, and heat transfer solids which stand out as exceptional in a relative sense, no particulate refractory oxide solid is now known which possesses the combination of properties which would render it a heat transfer solid capable of completely withstanding sintering, agglomeration and attrition to the desired degree at the extreme conditions encountered in commercial fluidized bed syn gas operations, particularly commercial fluidized bed syn gas operations at extreme hydrothermal conditions. Thus, there exists an acute need for further improving and maintaining the fluidization characteristics of the bed, or beds, employed in fluidized bed synthesis gas manufacturing. The present invention provides a process for the production of hydrogen and carbon monoxide by contacting one or more low molecular weight hydrocarbons and steam and/or oxygen with a fluidized bed at high temperatures known for such process: wherein the fluidized bed comprises catalyst and/or heat transfer solids, of fluidizable particle size distribution, of particulate alumina in which the alumina is substantially entirely alpha alumina, and wherein the catalyst and/or heat transfer solids are modified by a metal composited-therewith, the overall composite particle being represented by the formula: M x A1 2 O 3+x with the core of the particle being represented by the formula: M y A1 2 O 3+y where in formulae (1) and (2) M is a Group IIA metal, x is from 0.01 to 0.4 and is representative of the number of moles of the metal M per mole of alpha A1 2 O 3 , y is equal to or greater than zero, and x is greater than y. The core and the outer layer of a particle are distinguishable, the points of transition between the core and outer layer of a composite particle being defined as those locations where x becomes greater than y. The particulate solids forming the catalyst support, or heat transfer solid, is of fluidizable particle size distribution, and of average diameter ranging from 30 microns to 150 microns, preferably from 60 microns to 90 microns. The solids particles constituting the catalyst support, or heat transfer solid can be formed as discrete solids particles or as aggregates wherein a plurality of the solids particles are collected and held together in a mass of size representative of the sum-total of the composite of the individual particles of the mass. Generally, particles of average diameter greater than 0.01 microns, preferably of average diameter ranging from 0.1 microns to 25 microns, are used in forming the aggregates; and preferably the average diameters of the aggregate particles range from 30 microns to 150 microns, preferably from 60 microns to 90 microns. In accordance with a feature of the invention, the process employs a catalyst composition comprising the particulate structurally modified alumina solids supra, composited with a catalytic metal, or metals component, selected from one or more of nickel, iron, platinum, ruthenium. Of the catalytic metals nickel is preferred. Preferably, the heat transfer solid is prepared by impregnation of a preformed particulate alumina, or alumina previously dried and calcined after having been extruded, pressed, pilled, pelleted, beaded, then crushed, ground and classified, spray dried or otherwise formed and shaped to the desired particle size distribution and then contacted with a solution, or solutions, of a compound or salt of a Group IIA metal, or metals, to deposit the compound or salt onto the outer surface of the particles to form the composite particles described by formulas (1) and (2), supra. US-A-2, 422,172 discloses alkaline earth metal modified beta alumina compounds; molybdenum catalysts prepared therefrom; and the use of such catalysts in the process of catalytically hydroreforming a normally liquid hydrocarbon under process conditions which include a temperature in the range 425 to 550°. WO 81/01284 and EP-A-0119933 both disclose alumina supported copper catalysts and their use in a fluidized bed process for the synthesis in the range 200 to 250°C of 1-2 dichloroethane by the oxy(hydro)chlorination of ethylene. US-A-4,069,170 also discloses fluidized bed oxychlorination of aliphatic hydrocarbons, the catalyst employed comprising an alpha alumina support carrying copper chloride and certain rare earth chlorides. The catalyst may also contain magnesium chloride as a caking-inhibitor at process start-up or shut-down. In the Accompanying Drawings Figure 1 depicts in cross-section a particle of tabular alpha alumina structurally modified with magnesium, examined by scanning electron microscopy (SEM) as described by reference to Example 5. The microprobe analysis across the diameter of the particle shows that the magnesium is concentrated near the outer surface of the particle. Figure 2 depicts in cross-section a particle of tabular alpha alumina modified with barium as described by reference to Example 5. The particle is viewed via sanning electron microscopic analysis; the presence and location of the barium being enhanced via energy dispersive x-ray (SEM-EDX) analysis. Again, it is shown that the barium is dispersed near the outer surface of the particle. It has been found that structural modification of the outer surface of the catalyst support or heat transfer solids particles with an effective amount of the Group IIA metal, or metals, as characterized in (a), supra, lessens or suppresses the normal tendency of the particles to sinter and agglomerate even when the particles contain surface impurities in concentrations sufficient to promote, or produce sintering and agglomeration of the particles at normal syn gas operating conditions. Thus, e.g., surface modification of the particles with the Group IIA metal, or metals, lessens, inhibits, or suppresses the normal tendency of the particles to sinter and agglomerate at syn gas operating conditions even when the outer surface of the particles contain such known impurities as silicon and boron. Exemplary of the metals useful for the practice of this invention in forming the catalyst support or heat transfer solid as characterized in (a), supra, are magnesium strontium and barium, of which magnesium and barium are particularly preferred. These metals can be surface composited onto the alumina in effective amounts generally via such methods as ion-exchange, impregnation, spray-coating, spray drying and the like; suitably from a solution, or solutions, preferably from an aqueous solution of a compound or salt of the metal, e.g., magnesium nitrate, magnesium oxalate, magnesium acetate, barium carbonate, magnesium chloride, or the like. An alumina meeting the requirements of formulas (1) and (2), supra, is one wherein the concentration of the Group IIA metal, or metals, is greater at, or near, the outer or exterior surface of the solids particles (as opposed to the interior surface); and greater at the outer or exterior surface than present throughout the total of the particulate solids. The surfaces of the particulate alumina solids meeting the requirements of formulas (1) and (2), in other words, are structurally modified by the addition thereto of a relatively high concentration of the Group IIA metal, or metals, whereas in contrast a relatively small concentration of the Group IIA metal, or metals, if any, is added to the core, or center of the particles. The outer surface of an alumina particle is thus provided with a rim, coating, or outer surface layer surrounding a core, within which the Group IIA metal, or metals, is present in greatest concentration. The overall particle morphology of the parent alumina is largely retained, but the Group IIA metal modified particles display significant differences relative to the parent alumina in high temperature oxidizing, reducing and hydrothermal environments. The Group IIA metal modified particles provide greatly improved resistance to sintering and agglomeration, particularly in fluidized bed syn gas manufacturing operations. The tendency of the Group IIA metal modified particles to sinter and agglomerate at fluidized bed syn gas processing conditions is greatly reduced vis-a-vis particles otherwise similar except that they are untreated and are subjected to similar processing conditions. Whereas the mechanism for this phenomenon is unknown it is possible that the Group IIA metals form surface compounds which exhibit increased melting points relative to the untreated particulate oxide solid. The Group IIA metal, or metals, as suggested, can be added to the surface of the alumina particles by various methods, suitably, e.g. by ion-exchange, impregnation, or spray-coating the Group IIA metal, or metals, onto the surface of the particles. In the ionexchange and impregnation methods the preformed particulate alumina solids are contacted with a solution, or solutions, suitably an aqueous solution containing the desired Group IIA metal compound, or salt, and ion-exchanged or the solution imbibed into the particles. In the impregnation method, e.g., particulate alpha alumina solids are immersed and soaked in a solution which contains a salt or soluble compound of the Group IIA metal, suitably a nitrite, nitrate, acetate, carbonate or the like of magnesium, or barium, for a time sufficient to impregnate sufficient of the solution into the outer surface of the particles to provide the desired metallic loading. The particulate solids are then dried, and calcined. If more than one Group IIA metal is to be introduced into the solids, solutions containing salts of each metal may be introduced simultaneously, or sequentially by treatment with different solutions. Spray coating can be used to coat a preformed alumina by spraying the heated particles with a heat decomposable Group IIA metal compound, or compounds, to deposit and coat the solids particles with said Group IIA metal, or metals. A spray drying technique can be employed by preparing a slurry of alpha Al 2 O 3 solids, admixing same with a binder and a solution of the desired Group IIA metal compound, or salt, to form a sprayable paste, and spray drying the paste to form a fluidized bed of the composite particles. Composite particles of this type are agglomerates, the individual component particles of which, though affixed together, are surface coated with the Group IIA metal, or metals, to conform with formulas (1) and (2), supra. The agglomerated particles are then dried and calcined. Alternatively, the alumina paste can be spray dried without addition of the Group IIA metal compound, or salt, to form a particulate alumina, the composite alumina particles calcined, and the particles then impregnated with the Group IIA metal, or salt, and the particles then dried, and again calcined. Hollow spheres of fused alumina, or shells of spray dried alumina surrounding a void space, the solids particles portion of which has been surface coated with the Group IIA metal, or metals, have been found particularly useful as catalyst supports, or heat transfer solids for fluidized bed syn gas operations. In certain of its embodiments, the present invention employs a heat transfer component, or an admixture of, as a heat transfer component, the Group IIA metal modified particulate alumina and a catalyst. The particles are of fluidizable sizes, the average particle diameter of both the heat transfer component and catalyst ranging from about 30 microns to about 150 microns, preferably from about 60 microns to about 90 microns. The catalyst component of the admixture can be a support, or carrier, notably a refractory inorganic oxide, particularly alumina, with which is composited a metal, or metals, e.g., nickel, iron, ruthenium platinum, or the like, catalytic for the production of hydrogen and carbon monoxide from low molecular weight hydrocarbons contacted with a fluidized bed of the catalyst at high temperature hydrothermal conditions. Suitably the catalyst is a nickel-on-alumina catalyst, and preferably a nickel-on-alpha alumina catalyst, of particle size distribution corresponding to that of the heat transfer component. The preferred catalyst for use in said admixture, is one comprising a composite of one or more of said catalytic metals, preferably nickel, and a particulate Group IIA metal modified alumina support. The catalyst employed in the process of this invention contains preferably from about 1 percent to about 20 percent nickel, more preferably from about 5 percent to about 10 percent nickel, composited with the Group IIA metal modified alumina support, preferably a Group IIA metal modified alpha alumina support. An admixture constituted of from about 10 percent to about 99.9 percent, preferably from about 80 percent to about 99.5 percent, of said particulate heat transfer solid, and from about 0.1 percent to about 90 percent, preferably from about 0.5 percent to about 20 percent, of a catalyst, based on the total weight of the admixture, can be fluidized at temperatures ranging to about 2600°F (1427°C), particularly at temperatures ranging between about 1600°F and 2000°F (871°C and 1093°C), and contacted with a low molecular weight hydrocarbon, e.g., C 1 -C 4 alkanes, predominantly methane, in the presence of steam, or oxygen (air), or both steam and oxygen (air), to produce syn gas without significant sintering and agglomeration of the particulate solids components of the admixture. There is no significant disintegration of the catalyst or of the heat transfer solids component of the admixture to fines, or degradation of the catalyst and heat transfer solids particles of the admixture to agglomerates. In short, in the process of this invention both the catalyst employed and heat transfer solids employed are highly resistant to sintering, attrition and agglomeration per se, and the heat transfer solids impart high resistance to sintering and agglomeration to even the more conventional catalysts when used in said admixtures. Certain features of the bad particles employed in the process of the invention, will be better understood via reference to the following examples which illustrate specific and preferred embodiments. All parts are in terms of weight except as otherwise specified. Example 1 Several specimens of alumina, spanning a wide range of initial structure and morphology, of nominal purity greater than 98.5 percent, of particle diameter ranging between 45 and 106 micrometers, were obtained from several commercial suppliers: A, B, C, D, E, F and G. The alumina specimens, which were found to exhibit significant differences in agglomeration behavior in fixed bed sintering tests designed to simulate agglomeration tendency in fluidized bed synthesis gas generation, are listed in Table 1. Starting Alumina Materials for Structural Modification Alumina Initial Structure Morphology A calcined alumina trihydrate granular/microporous B alpha alumina tabular C alpha alumina spray dried D alpha alumina spray dried E alpha alumina hollow spheres F alpha alumina hollow spheres G alpha alumina tabular The particulate alumina specimens, in portions weighing about 50100 gms, were structurally modified with Group IIA metals. The alumina specimens were thus impregnated with aqueous solutions of Group IIA metal salts, i.e., magnesium nitrate, strontium nitrate and barium nitrate, using the incipient wetness impregnation technique, followed by drying at ambient temperature for 16-36 hours, drying in vacuum at 100°C for 16-36 hours, and calcination in air at 1000°C to 1200°C for at least four hours. Loadings ranged in the 1% to 9% range, based on the weight of metal contained within the total specimen (dry basis); loadings below the amount of metal required to form a metal aluminate, MAl 2 O 4 . Example 2 Each of the starting aluminas, for comparative purposes, and corresponding Group II structurally modified aluminas were investigated for agglomeration resistance in a fixed bed sintering test. In this test, an 8-10 gr sample of the 45-106 µm particulate oxide was distributed in a small Coors alumina boat. The sample was placed in a high temperature furnace and heated from room temperature to a maximum temperature of 3000°F (1649°C) over a period of several hours. The sample was then held at the maximum temperature for 2 to 16 hours to induce thermal sintering and agglomeration. The sample was then cooled to about 200°F (93°C) over a period of 6-12 hours and removed from the oven. The sample was then transferred to a sonic sieve operated at a constant power level, and the conversion of 45 to 106 µm particles to fused aggregates greater than 106 µm in size was measured by weighing the fraction collected on a 150 mesh size screen. Table 2 summarizes the agglomeration results for the various materials investigated. In essentially all cases, it is seen that addition of the Group II metal resulted in significant improvements in alumina agglomeration resistance. Agglomeration Results for Structurally Modified Aluminas Agglomeration, Wt% > 106 µm Formed Material 3 Hours At 3000°F (1649°C) 16 Hours At 3000°F (1649°C) A 20 -- 4% Mg/A 9 -- 8% Mg/A 9 -- B 17 -- 2% Mg/B 0.8 -- 2% Ba/B 2 -- C 0.6 30 3% Mg/C 0.5 33 3% Ba/C 0.3 1.4 D 3.4 -- 3% Mg/D 0.4 -- 3% Ba/D 0.2 -- E 40 72 1% Mg/E 11 81 2% Mg/E 0.8 20 3% Mg/E 1.6 26 1% Ba/E 1.2 39 3% Ba/E 1.2 0.4 2.2% Sr/E 2.9 3.6 4.3% Sr/E 1.4 1.6 8.6% Sr/E 1.5 2.0 F 31/20 62 3% Mg/F 4 62 4% Mg/F 6 -- 3% Ba/F -- 0.7 G 90 96 1.6% Mg/G 2 28 2.2% Sr/G 0.4 0.4 4.3% Sr/G 0.6 0.6 8.6% Sr/G 1.2 1.8 Example 3 Tests were conducted using high purity magnesium aluminate spinel materials obtained from appropriate commercial manufacturers; H, I, J, K and B. These were tabular materials with uniform magnesium concentrations throughout the particle that were produced by co-firing aluminum and magnesium oxide gels at temperatures above 3000°F (1649°C) prior to grinding to produce the particulate spinel materials. Table 3 summarizes agglomeration results for these prior art materials in comparison to an alpha alumina which was structurally modified by Mg-impregnation. The tabular spinels provided inferior agglomeration resistance, while high agglomeration resistance is provided by the doped alumina of manufacturer B. Agglomeration Behavior of Magnesium Aluminate Spinels Material Mg-Content (Wt%) Agglomeration, Wt% > 106 µm @ 3000°F (1649°C) H - spinel 17 94 I - tabular spinel 6 33 J - tabular spinel 13 98 K - tabular spinel 20 96 B - tabular alumina -- 17 2% Mg/B-tabular alumina 2 1 Example 4 Examination of the data given by Tables 2 and 3, plotting relative agglomeration resistance versus magnesium loading suggests a generally optimum level of magnesium addition. If the composite material is represented by the formula Mg x Al 2 O 3+x , this optimum appears to occur for values of x in the range of about 0.07 to 0.14. As shown in Table 2, strontium shows a similar optimum. It is expected that barium would show generally similar results. Example 5 The structurally modified aluminas were characterized using a variety of methods such as powder X-ray diffraction and various types of electron microscopy. X-ray diffraction results showed that the impregnation of Group IIA metal salts followed by calcination resulted in the appearance of new diffraction features. In general, the positions of these new diffraction lines agreed closely with those expected for spinel compounds such as Mg x Al 2 O 3+x , Ba x Al 2 O 3+x , etc. However, electron microscopy data also revealed that the Group II additive was characterized by a non-uniform (rim-type) distribution where most if not all of the Group II additive was concentrated in the near, or outer surface region of the particle or in the surface region of the individual grains which comprised composite spray dried particles. Referring to Figure 1, e.g., there is depicted in cross-section a particle of tabular alumina modified with magnesium, as it appears via scanning electron microscopic analysis. Also, there is depicted in Figure 2 an exemplary electron micrograph for a barium doped tabular alumina. It is apparent from these elemental "maps" that the Group IIA metals, e.g. barium, strontium and magnesium, are concentrated in a rim like surface layer and not uniformly distributed throughout the bulk of the particle. The following exemplifies the effect of magnesium impregnated into alpha alumina spheres containing titanium and calcium impurities as regards the suppression of the alpha alumina to agglomerate at high temperature. Example 6 Mg was impregnated to a 0.7 wt% level onto alpha alumina spheres containing 0.5 wt% Ti and 0.5 wt% Ca as impurities. The tendency of the Mg-impregnated alpha alumina particles to agglomerate, utilizing the test procedure described in Example 2, was compared with particles similar in all respects except that the particles were not impregnated with Mg, i.e. a stock sample, as shown in Table 4. The high agglomeration of the stock sample, as shown by these comparative data, results from the presence of Ti and Ca impurities. The addition of Mg, as is apparent, counteracts their effect to give a reduced agglomeration. Mg Addition Reduces Agglomeration Agglomeration at 3 Hours, Wt% > 106 µm Sample 2800°F (1538°C) 2900°F (1593°C) 3000°F (1649°C) Stock 93 95 99 0.7 Wt% Mg on Stock 2 28 69 The procedure used for the agglomeration test is as described in Example 2.

Claims: A process for the production of hydrogen and carbon monoxide by contacting one or more low molecular weight hydrocarbons and steam and/or oxygen with a fluidized bed at high temperatures known for such process: wherein the fluidized bed comprises catalyst and/or heat transfer solids, of fluidizable particle size distribution, of particulate alumina in which the alumina is substantially entirely alpha alumina, and wherein the catalyst and/or heat transfer solids are modified by a metal composited-therewith, the overall composite particle being represented by the formula : M x A1 2 O 3+x with the core of the particle being represented by the formula : M y A1 2 O 3+y where in formulae (1) and (2) M is a Group IIA metal, x is from 0.01 to 0.4 and is representative of the number of moles of the metal M per mole of alpha A1 2 O 3 , y is equal to or greater than zero, and x is greater than y. The process of Claim 1, wherein the Group IIA metal is magnesium, strontium or barium. The process of Claim 1 or Claim 2, which employs as a catalyst said metal-modified solids further containing one or more of nickel, iron, platinum and ruthenium. The process of Claim 3, wherein the metal is nickel. The process of Claim 4, wherein the nickel is present in an amount from 1 percent to 20 percent, based on the total weight of the composite. The process of Claim 5, wherein the nickel concentration of the catalyst is from 5 percent to 10 percent. The process of any preceding Claim, wherein the composite particles are of average diameter from 30 to 150 microns, preferably from 60 to 90 microns. The process of any preceding Claim, wherein the fluidized bed contains an admixture of, based on total weight of the admixture, from 80 to 99.5 wt.% of the heat transfer solids and from 20 to 0.5 wt.% of the catalyst; and wherein the bed is fluidized at a temperature of from 1600°F (871°C) to 2600°F (1427°C).

Inventors: CLAVENNA LEROY RUSSELL, DAVIS STEPHEN MARK, FIATO ROCCO ANTHONY, SAY GEOFFREY RHYS, CLAVENNA, LEROY RUSSELL, DAVIS, STEPHEN MARK, FIATO, ROCCO ANTHONY, SAY, GEOFFREY RHYS
Applicants: EXXON RESEARCH ENGINEERING CO, EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1994
Main classification: B01J23
IPC codes: B01J35, B01J23